The use of change_hat when in complain mode can cause a deadlock
when the hat doesn't exist and a new learning profile is created
for the missing profile. This is because change_hat() has taken
the lock to search the hat list and creating the new learning
profile needs to take the lock to add it to the list.

From the bug report:

Originally found in 7.0.0 in LTS ubuntu 26.04 with pam_apparmor + su
in complain mode set to change hats.  Then verified in newest
available vanilla kernel I've compiled to see if still present:

7.2-rc7 vanilla -> affected

checked also some other kernels:
6.18.44 vanilla -> affected
6.12.95 with debian patches -> unaffected

On systems without bug (for example 6.12.95 debian) it just prints:

aa_change_hat rc=0

On systems with bug, the executable always hangs, prints nothing and
becomes unkillable.  (And once stuck this way, it will cause any
further hat changes to also cause the changing process to get stuck)

Then in syslog you can find hint about cause:

kernel: INFO: task hat:3409 blocked for more than 483 seconds.
kernel:       Not tainted 7.2.0-rc7 #1
kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kernel: task:hat             state:D stack:0     pid:3409  tgid:3409  ppid:2605   task_flags:0x400000 flags:0x00080800
kernel: Call Trace:
kernel:  <TASK>
kernel:  __schedule+0x48f/0xfe0
kernel:  schedule+0x27/0xa0
kernel:  schedule_preempt_disabled+0x15/0x30
kernel:  __mutex_lock.constprop.0+0x569/0xa10
kernel:  aa_new_learning_profile+0x15f/0x210
kernel:  build_change_hat+0x19f/0x3b0
kernel:  change_hat.isra.0+0x5dd/0xd60
kernel:  aa_change_hat+0x2f3/0x710
kernel:  aa_setprocattr_changehat+0x121/0x1f0
kernel:  do_setattr+0x28c/0x340
kernel:  apparmor_setselfattr+0x20/0x50
kernel:  security_setselfattr+0xf6/0x110
kernel:  __x64_sys_lsm_set_self_attr+0x53/0x90
kernel:  do_syscall_64+0xdd/0x5e0
kernel:  ? __mod_memcg_lruvec_state+0xfd/0x260
kernel:  ? lruvec_stat_mod_folio+0x8d/0xd0
kernel:  ? __folio_mod_stat+0x2d/0x90
kernel:  ? map_anon_folio_pte_nopf+0xd1/0x1f0
kernel:  ? do_anonymous_page+0x184/0xa10
kernel:  ? __handle_mm_fault+0x805/0x870
kernel:  ? count_memcg_events+0xef/0x230
kernel:  ? handle_mm_fault+0x1f0/0x2f0
kernel:  ? do_user_addr_fault+0x2bb/0x7b0
kernel:  ? do_syscall_64+0x94/0x5e0
kernel:  ? exc_page_fault+0x75/0x160
kernel:  entry_SYSCALL_64_after_hwframe+0x76/0x7e
kernel: RIP: 0033:0x7f815e134c8d
kernel: RSP: 002b:00007fff6df94ea8 EFLAGS: 00000246 ORIG_RAX: 00000000000001cc
kernel: RAX: ffffffffffffffda RBX: 0000556d8c81d040 RCX: 00007f815e134c8d
kernel: RDX: 0000000000000046 RSI: 0000556d8c81d040 RDI: 0000000000000064
kernel: RBP: 00007fff6df94ef0 R08: 00007f815e212ac8 R09: 000000000000000c
kernel: R10: 0000000000000000 R11: 0000000000000246 R12: 0000556d8c81d010
kernel: R13: 0000000000000026 R14: 0000000000000046 R15: 0000000000000064
kernel:  </TASK>
kernel: INFO: task hat:3409 is blocked on a mutex likely owned by task hat:3409.

To fix the issue, lift the locking out of the core of
aa_new_learning_profile(), introduce a wrapper function that takes the
lock where needed, and have build_change_hat() call the core function
that no longer takes the lock.

In addition fix 4 other issues introduced by commit
32e92764d6f8d ("apparmor: grab ns lock and refresh when looking up changehat child profiles")
- aa_get_profile_rcu() was replaced-by: aa_get_profile without the
  accompanying rcu_dereference_protected()
- an extra aa_get_label(label) was introduced at the start of
  change_hat() without an accompanying aa_put_label() causing a
  reference count leak.
- a reference count leak was introduced in the label_is_stale(label)
  case, where the newest profile would be leaked instead of the
  label passed to the function.
- a potential UAF when the lookup walks up the tree with new_ns != ns
  the new label reference is put, and then used for the next lookup.
  The mutex_lock, will block replacement, and removal in the locked
  ns. However there are two cases where putting the reference can
  result in the label being freed even with the lock held.

  1. the label does not have a list reference (possible for temporary
     or special profiles) in which case the put can trigger the
     cleanup.
  2. the new label reference is in a different namespace, which does
     not have a lock held on it. This extends case 1 to also include
     replacement, and removal that could be occurring in the namespace
     new is in.

Reported-by: Martin Petricek <mp@petricek.net>
Link: https://lists.ubuntu.com/archives/apparmor/2026-August/014907.html
Fixes: 32e92764d6f8d ("apparmor: grab ns lock and refresh when looking up changehat child profiles")
Signed-off-by: John Johansen <john.johansen@canonical.com>

Several label helpers only read from their struct aa_label * arguments:
they compare labels, test subset relationships, or check the mediation
bitmask, all via direct field/index access. Mark those parameters
const struct aa_label * to document intent and let the compiler enforce
that the label is not modified.

The converted functions are:
  - label_mediates(), label_mediates_safe()
  - aa_label_cmp() (and its vec_cmp() helper)
  - __aa_label_next_not_in_set(), aa_label_is_subset(),
    aa_label_is_unconfined_subset()
  - __aa_subj_label_is_cached()
  - aa_label_next_confined(), aa_label_next_in_merge()

These all access the label through direct indexing or manual iterators
rather than the label_for_each()/fn_for_each() macros, which are not
const-correct and so gate the majority of the remaining label consumers
(the print, match, and permission-check paths) from being constified.

No functional change.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Assisted-by: Claude:claude-opus-4.8

Most uses of aa_dfa a read-only walking of the dfa. Have the compiler
enforce this.

Signed-off-by: John Johansen <john.johansen@canonical.com>

A number of functions take a struct aa_profile * argument that is only
ever read from: they compute DFA matches or apply the profile's mode
flags without modifying the profile, taking a reference on it, or
touching its embedded label. Mark those parameters const struct
aa_profile * to document intent and let the compiler enforce it.

The converted functions are the permission "compute" path plus a few
pure readers:
  - aa_apply_modes_to_perms(), aa_profile_match_label()
  - AUDIT_MODE()
  - aa_label_match() and its match_component()/label_compound_match()/
    label_components_match() helpers (label.c)
  - match_component()/label_compound_match()/label_components_match()/
    label_match()/change_profile_perms()/aa_xattrs_match() (domain.c)
  - match_iface()/match_addr_iface()/match_addr_iface_label()/
    skb_match_to_sk()/skb_match_to_cmd() (af_inet.c)
  - aa_profile_capget(), path_flags(), profile_query_cb()

The remaining aa_profile * parameters cannot be made const: the audit
path stores &profile->label into the owned, refcounted
apparmor_audit_data.subj_label/peer fields, and the domain/lifecycle
paths take references on the profile's embedded label
(aa_get_label()/aa_get_newest_label()/aa_get_profile()) or write
profile fields.

No functional change.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Assisted-by: Claude:claude-opus-4.8

Several functions take a struct aa_perms * argument that is only ever
read from and never modified through the pointer. Mark those parameters
const struct aa_perms * to document intent and let the compiler enforce
that the permission set is not mutated.

The converted functions are:
  - aa_check_perms()
  - aa_do_perms()
  - do_perms() (af_inet)
  - match_label() (af_unix)
  - verify_perm()
  - aa_perms_accum() / aa_perms_accum_raw() (@addend only)

No functional change.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Assisted-by: Claude:claude-opus-4.8

Move the mount permissions check to use the common backend
aa_check_perms() to check permissions. This will make it so caching,
audit, complain, logic can be handled consistently in a single place.

Signed-off-by: John Johansen <john.johansen@canonical.com>

Complain messages can be very noisy and fill the logs quickly. Allow
complain (allow) messages to be silenced separate from denied
messages.

Signed-off-by: John Johansen <john.johansen@canonical.com>

static tables, and structs that are initialized as part of their
data section or during init should be read only to protect against
accidental or malicous changes.

Signed-off-by: John Johansen <john.johansen@canonical.com>

checking PTR_ERROR() is not correct to just determine if any error
occured, instead use the IS_ERR macro and also output the PTR_ERR
as part of the debug message.

Signed-off-by: John Johansen <john.johansen@canonical.com>

Instead of keeping an external define for the various tables indexed
by an enum, make the size the last entry of the enum so the table
size will get updated correctly with changes to the enum.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>